Generally, method of separating highly pure 1,3-butadiene from crude C4 stream, which is discharged from a cracker and contains acetylenes, uses the butadiene extraction unit (BEU) with two extractive distillation columns (EDC).
It is inevitable to lose the 1,3-Butadiene because it is used as a diluent to eliminate potential explosion of the vinylacetylene in conventional butadiene extraction unit.
In present extraction units, the bottom temperature of a 1,3-butadiene recovery column, which is used for recovering some 1,3-butadienes from the bottom stream of the 2nd EDC, is dependent on vinylacetylenes to 1,3-butadiene ratio of C4 feed stream of the 1st EDC. 1,3-Butadiene recovery can be improved by increasing operating temperature of 1,3-butadiene recovery column when their ratio is low. However, in case of the C4 stream with more than 1.2 wt % of vinylacetylene, the bottom stream of 1,3-butadiene recovery column should be diluted by means of adding 1,3-butadiene to avoid the explosion threshold of vinylacetylene. Thus, it is difficult to increase the temperature of 1,3-butadiene recovery column more than 130° C. So 1,3-butadiene recovery in overall process is only 96˜97%. Additionally, acetylene contamination of columns and heat-exchangers restricts long term operation of BEU.
To solve these problems, a method to remove acetylenes (particularly vinylacetylene) beforehand in BEU feedstocks has been proposed.
The crude C4 mixtures from a naphtha cracker is consisted of 0.5˜2.0 wt % of vinylacetylene, 0.1˜0.3 wt % ethylacetylene, and 0.01˜0.10 wt % methylacetylene. There are two methods of removing acetylenes through hydrogenation to convert acetylenes into 1,3-butadiene, 1-butene and propylenes, respectively, and BEU with two-EDCs.
The Handbook of Petroleum Refining Process (McGraw-Hill, 3rd ed., Chapter 8.2(8.25), 2004) discloses the process of producing highly pure 1,3-butadiene by integrating a KLP™ process with two or three hydrogenation reactors and BEU with only one EDC. The KLP™ process includes not only removing unit for sulfur compounds to prevent the deactivation of a Cu-based catalyst in the C4 mixtures but also a water washing unit. In this process, the recycled C4 mixtures are fed to the reactor along with fresh C4 mixtures and hydrogen. The hydrogenation is performed at 15 kg/cm2·g and 30˜50° C. by using two reactors. Recently, to increase the solubility of hydrogen, the reactor has been designed to be affordable for operation at 40 kg/cm2·g and 60˜70° C. Then the hydrogenation product is fed to the distillation column, so that the C4 stream is obtained from the bottom and a light gas component is discharged from the column overhead. However, this process has defect of requiring increased energy consumption because of using additional refrigerator to prevent loss of the C4 mixtures when the light gases are removed at column overhead.
There are some other conventional techniques which integrate selective hydrogenation of acetylenes in the C4 mixtures other than the KLP™ process and BEU for 1,3-butadiene purification. U.S. Pat. No. 4,277,313 includes a hydrogenation to remove acetylenes and BEU in which the overhead stream of the 2nd solvent recovery column is recycled into the hydrogenation reactor along with the C4 mixtures.
However, in this case, overhead stream of the 2nd solvent recovery column includes 1,3-butadiene in almost as the same amount to that of vinylacetylene along with a large amount of solvent. However, the recycle of overhead stream of the 2nd solvent recovery column is problematic in that the gas phase of it must be additionally condensed to increase the pressure before introducing it into the hydrogenation reactor because only gas phase of it is recycled.
The hydrogen used for hydrogenation should be completely dissolved in the C4 mixtures and the molar ratio of hydrogen to acetylene should be more than 1. However, hydrogen cannot be dissolved to the required ratio because hydrogenation in above patent is performed at 6 kg/cm2·g and 30° C. It is impossible to completely convert acetylenes in this condition. U.S. Pat. No. 6,040,689 suggests reactive extractive distillation column which integrates a hydrogenation with a Cu-based catalyst into an extractive distillation.
In order to connect the selective hydrogenation of acetylenes in crude C4 mixtures with dienes and the extraction process, the reaction stability of the acetylene converter, a long-term to operation and easiness of catalyst regeneration are required basically. Furthermore, in order to simplify the process and increase economical efficiency, only one EDC operation is preferable. To recover highly pure 1,3-butadiene by using only one EDC after the selective hydrogenation process, the loss of 1,3-butadiene and vinylacetylene concentration of hydrogenation effluent should be less than 1 wt % and less than 70 ppm respectively.